1. Field of the Invention
The present invention relates to a valve timing control apparatus that controls valve timing of at least one valve, which is driven by a camshaft through transmission of a torque from a crankshaft of an internal combustion engine.
2. Description of Related Art
A known valve timing control apparatus of a fluid drive type has a housing and a vane rotor. The housing serves as a driving-side rotator, which is rotated synchronously with a crankshaft. The vane rotor serves as a driven-side rotator, which is rotated synchronously with a camshaft. Japanese Unexamined Patent Publication No. 2006-63835 discloses this type of valve timing control apparatus, in which hydraulic fluid is supplied to advancing chambers or retarding chambers, each of which extends in a rotational direction and is defined between a corresponding shoe of the housing and a corresponding vane of the vane rotor, so that the camshaft is driven relative to the crankshaft in the advancing direction or the retarding direction to adjust the valve timing.
Here, in the valve timing control apparatus of Japanese Unexamined Patent Publication No. 2006-63835, a spool valve is used to change communication of a supply passage, into which the hydraulic fluid is supplied from a pump, to the advancing chambers or the retarding chambers. Specifically, at the time of changing the phase (hereinafter, referred to as an engine phase) of the camshaft relative to the crankshaft toward the advancing side, a port, which is communicated with the supply passage, is communicated with a port, which is communicated with the advancing chambers, by moving a spool of the spool valve to a corresponding position. Furthermore, at the time of changing the engine phase toward the retarding side, the port, which is communicated with the supply passage, is communicated with a port, which is communicated with the retarding chambers, by moving the spool to a corresponding position.
In the valve timing control apparatus of Japanese Unexamined Patent Publication No. 2006-63835, the variable torque is varied between the negative torque side for advancing the camshaft relative to the crankshaft and the positive torque side for retarding the camshaft relative to the crankshaft. Here, the variable torque is always applied during the operation of the internal combustion engine by, for example, a spring reaction force of the valves, which are driven by the camshaft. The amount of the variable torque changes depending on the rotational state of the internal combustion engine.
Therefore, in the case of changing the engine phase toward the advancing side, when the amount of supply of the hydraulic fluid from the pump is relatively small at the time of applying the negative torque as the variable torque, the hydraulic fluid becomes deficient in the advancing chambers, the volume of which is increased by the action of the negative torque. Thus, when the variable torque is reversed from the negative torque to the positive torque, the retardation of the camshaft cannot be limited due to the deficient of the working fluid. As a result, the response at the time of advancing the engine phase is disadvantageously deteriorated. The deterioration of the response also occurs at the time of changing the engine phase toward the retarding side. Therefore, it is desirable to take appropriate measures for both of the advancing side change and the retarding side change of the engine phase.